Abstract: An alarm for signalling an improperly high temperature in a refrigerated container such as a freezer includes a thermostat within the container and an alarm unit magnetically mounted on the outside of the container. The alarm unit includes batteries for energizing a lamp and a buzzer within the unit in response to the thermostat. The alarm unit also includes a spring biased flag pivotly mounted thereon which is normally retained in a cocked position by a finger carried by a solenoid. The solenoid is energized by the battery to deploy the flag and thereby provide an enduring indication of alarm after the batteries have died out or the temperature has returned to normal.